Mitchel Lichtman uses the bottom of the eighth of ALCS game six to discuss bunting strategy. He wants managers to keep the defense guessing:

There are two ways you can actually answer the question – or at least the question, “Did the manager know what he was doing in this situation or on sacrifice bunts in general?” One, you can ask him what his intention was when he gave the sac bunt signal. If he says something like, “Well, in that situation sometimes I do and sometimes I don’t, and I just happened to (randomly) choose the former this time,” you know that he was right on track. You still don’t know how often he was planning on bunting and whether those percentages are correct or not, but, trust me, if a manager were to say that, he is probably a pretty smart cookie. Two, you can observe the same or at least a similar situation over many, many games and if you see that he sometimes bunts and sometimes doesn’t and you can’t really tell beforehand which way he is going to go (i.e. it appears that his decision is random), then again, he is on the right track and you have a very smart manager, at least with respect to the sac bunt. Using this method (observing many games) is better because now you can get an idea as to how often he bunts or doesn’t bunt in any given situation and compare that to the optimal percentages (if you knew what they were, which is not so easy to figure out).

Basically, most situations in which a manager might sacrifice do not have 100% chances of improving the situation. By being unpredictable, the offensive manager can keep the defense guessing, and gain an advantage in by forcing the defense to play half-way between stopping a bunt and stopping a the result of a normal swing.